The present invention relates to electrochemical processes for production of chlorine and caustic from brine and to the simultaneous production of electrical energy. More particularly, the invention is directed to the treatment of cell liquor from a chloralkali cell to separate the sodium ions from the cell liquor and concentrate them in another liquor as a sodium hydroxide solution.
The production of chlorine and crude caustic solutions by electrolysis of brine is a major industry. Two types of electrolysis cells are primarly used in the production of chlorine and caustic. They are the diaphragm cell and the mercury cell. Membrane cells are also used to a minor but growing extent. Considerable quantities of energy are required for electrolysis of the brine to produce chlorine and subsequent treatment of the cell liquor resulting from electrolysis in diaphragm cells is necessary to obtain caustic solutions of the desired purity and concentration. A 50 weight percent aqueous caustic solution of low sodium chloride content is a commercially desired product.
Known processes for electrolysis of brine in diaphragm mercury and membrane cells produce cathode cell liquors having a caustic content of about 10 to as high as about 40 percent by weight in membrane cells and 50 percent by weight in mercury cells. Sodium chloride content of the liquor is up to about 15 percent by weight for diaphragm cells, virtually absent in the liquor of membrane cells and essentially absent in the liquor of mercury cells. Mercury cells have environmental problems and are no longer the technology of choice in industrialized countries. The cathode cell liquor produced by a diaphragm cell typically contains about 10 to 12 percent by weight caustic (NaOH) and 15 percent by weight sodium chloride (NaCl).
In the diaphragm cell, brine is continuously fed to an anode compartment, where chlorine is produced, and then flows through a diaphragm, usually made of asbestos, to a cathode compartment. Hydrogen gas is discharged from the solution at the cathode, with attendant generation of hydroxyl ions. To minimize back-migration of hydroxide ions from the cathode compartment to the anode compartment, a positive flow rate is always maintained; that is, a flow in excess of the conversion rate. As a consequence, the resulting catholyte solution, i.e., the cathode cell liquor as the term is used herein, has unconsumed sodium chloride in addition to product sodium hydroxide. The cathode cell liquor containing the sodium hydroxide and sodium chloride must be purified and must be concentrated to obtain a salable caustic solution.
A membrane cell, which employs a membrane selectively permeable to certain cations in place of a diaphragm, yields a catholyte of low salt content and having a caustic content of up to about 40 percent by weight. The highly corrosive chlorine medium, however, is harsh on membrane materials. Accordingly, specifications for the membrane must be rigid and the membranes useful in the presence of chlorine are quite expensive. In addition, voltage drop within the membrane cell is relatively high which increases consumption of electricity. In sum, membrane cells are costly in regard to investment and operating costs.
Typical processes for concentrating cell liquor and separating the sodium chloride from the caustic involve evaporation and crystallization with the consumption of large amounts of steam and, consequently, fuel required to generate steam. Investment in such processes is considerable.
The hybrid cell, described in greater detail herein, is an electrochemical generator of the fuel cell type consuming hydrogen at the anode and oxygen at the cathode. It includes an additional integrated electrodialysis function. The electrolytic space is separated into two sub-spaces by a diffusion barrier, with the anolyte on one side and the catholyte on the other. These electrolytes pass through the cell parallel to the plane of the diffusion barrier. The function of a hybrid cell is the supply of electrical energy and the exhaustion of a chemical species contained in the anolyte as well as the accompanying enrichment of the catholyte with the same chemical species. Typically, the anolyte is a 10% NaOH and 15% NaCl solution coming from a chloralkali cell to be exhausted to 0.5% or less NaOH, and the catholyte is a 0 to 10% NaOH solution at inlet and is enriched to 40% or more NaOH at the outlet.
Several hybrid cells electrically in series or supplied electrolytically in parallel, may have associated with them various circulation modes for the anolyte and catholyte, namely, ascending cocurrent, descending cocurrent, and countercurrent. All of these circulation modes have drawbacks.
The significant variation in the anolyte concentration (for example, from 10% to 0.1% NaOH) requires the entire anode to work at the lowest potential corresponding to the part of the solution most diluted with respect to the chemical species to be exhausted. The hydroxide concentration is smallest in the terminal passage region of the anode compartment and tends to establish the potential of the entire anode. As a consequence polarization of the anode is increased and voltage efficiency of cell is reduced.
As concentration gradients increase across the diffusion barrier, chemical driving forces may promote back-diffusion of the caustic product from a high-strength catholyte to the lower-strength anolyte, which reduces the concentration of alkali metal hydroxide in the produce and the overall efficiency of the process.
Many commercially available cation permselective diffusion barriers exhibit a permselectivity which decreases as the difference in concentration on each side of the diffusion barrier increases.
The above-mentioned phenomena may be minimized by countercurrent circulation of anolyte and catholyte. In contrast, cocurrent circulation would aggravate these undesirable effects. However, countercurrent circulation has its disadvantages. The exhustion of the anolyte and the enrichment of the catholyte require a slow electrolyte circulation rate requiring a high degree of control of the flow with plug flow being preferred to prevent back mixing and disruption of the concentration gradient. Cocurrent circulation best maintains a condition of plug flow through the cell compartments. Countercurrent flow, by contrast, leads to the construction of a hybrid cell with components having very close tolerances, and hence of high cost. Moreover, cocurrent flow minimizes the difference in pressure on each side of the diffusion barrier compared with countercurrent and consequently reduces any cross-diffusion related to membrane imperfections, such as holes, for example. Countercurrent flow would aggravate the problems of cross-diffusion.
The above-mentioned drawbacks of the cocurrent and countercurrent circulation modes are minimized by the use of the cascade of this invention, which is described more fully herein.